His comments came during a meeting with Jordan's King Abdullah in the Oval Office.

The US President, referring to a suspected chemical attack, said: “These are very troubled times in the Middle East. And we see what happened just yesterday in Syria. Horrible, horrible things, unspeakable.”

It comes as the US envoy to the UN said America could be "compelled to take its own action" in Syria if the world body fails to act.

"When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action," US Ambassador to the UN Nikki Haley told a Security Council meeting on Syria.

Holding up photos of victims of the alleged attack, she accused Russia of blocking action.

She said Moscow had closed its eyes to the "barbarity" of previous chemical attacks by vetoing a resolution in late February that would have imposed sanctions on those responsible.